Prestressed concrete bridges built in 1950s have deteriorated due to
prestress loss, aging, and corrosion. If load rating criteria are not met,
these bridges may be load posted or completely deconstructed. Accurate
estimation of prestress loss and assessing the condition of in-service girders
is vital, given the high cost of replacements. An example is North Carolina’s
Herbert C. Bonner Bridge, which was deconstructed after 56 years in service.
Four recovered AASHTO Type III girders were tested in the lab.
The
results of the experiments and analyses were used to make recommendations on
the performance of these girders, including discussion on prestress losses for
girders with and without corroded strands, on improved prestress loss
calculation methods, and on the remaining safe load carrying capacity of the
selected bridge girders. Testing involved a small number of load cycles at low
levels prior to montonically loading to ultimate capacity. The measured
prestress losses for the four specimens were 44.3 ksi, 34.0 ksi, 35.4 ksi, and
36.0 ksi. Sectional analysis models of the girders were developed in
Response-2000, which predicts the ultimate capacity and moment-curvature
response with reasonable accuracy.
Load Testing
The study has shown corrosion of strands can significantly influence effective prestress losses, and models developed in Response-2000 can capture these effects by accounting for the corroded strands. The study recommends the use of Response-2000 to model the behavior of aged prestressed concrete girders. At the time of replacement, the measured losses in the girders were such that the zero stress limit was exceeded under the service limit state according to the AASHTO LRFD Refined Method (using nominal material properties), the AASHTO LRFD Refined Method using measured material properties, Response-2000 with measured material properties and experimental measured values. In-situ material properties would ideally be used near the end of the service life to refine loss estimates, but it is not recommended at the design phase.
The study recommends that the bridge girders could have operated safely under an extended service life if the zero tensile stress limit could have been relaxed under the service limit states, only at the end of the service life. The results indicated that the experimental flexural strengths far exceeded the service limit states, which is safe and conservative.
